The Foundations: Classical Split and Splitless Injection

Nicholas H. SnowDepartment of Chemistry

Seton Hall UniversitySouth Orange, NJ 07079

snownich@shu.edu

Split and Splitless

• Split– vaporize and remove most of the sample to

waste

• Splitless– vaporize and transfer most of the sample to the

column; use cold trapping and solvent effects to focus bands

• Both use the same hardware

Split Inlet

• Use for higher concentration samples

• ppm and above

• hot inlet; vaporize sample

• mix with carrier gas

• use purge valve to “split” the sample– split ratio is critical

• place fraction of sample on column

SPLIT INJECTION

• High Temperature• High Linear Velocity• Rapid Transfer• Bulk of Sample

Wasted• Split Ratio Important• Liner Geometry

Classical Split Ratio Determination

• Measure column flow from tm

– Fc = r2L/tm

• Measure purge vent flow using flow meter– Fs

• Split Ratio = Fs / Fc

What are the problems with these measurements?Do we really ever know how much we injected?Does the exact injection volume matter?

Modern Split Ratio Determination

• EPC systems measure pressures and flows directly

• Column flow is calculated from inlet conditions and column dimensions– add equation here

• Purge flow adjusted to desired value

Flow Equations

Advantages of Split Inlets

• Reduced sample size (narrow bands)

• Fast inlet flow rate (narrow bands)

• Dirty samples OK

• Simple to operate (best for isothermal GC)

• Inject “neat” samples

• Excellent interfacing

Disadvantages of Split Inlets

• Nonlinear splitting– high molecular weights can be lost preferentially

• Thermal degradation – hot metal surfaces can lead to reaction

• Syringe needle discrimination

• Trace analysis limited – ppm detection limits with FID

Split Injection Techniques

• Filled Needle

• Cold Needle

• Hot Needle

• Solvent Flush

Split Inlet Discrimination

Summary - Split Inlet

• Simple

• Hot vaporizing technique– syringe discrimination (best to use autosampler)– liner discrimination

• use glass wool (deactivated)

• shape of liner may be critical

• Best for “neat” or concentrated samples– high ppm or higher

Splitless Inlet

• Inject sample into hot inlet without “purge”• 95% of sample enters column• Same hardware as split except liner• More variables

– solvent, splitless time, initial column temperature

• Open purge valve after short time• Better sensitivity

SPLITLESS INJECTION

• High Temperature• Low Liner Velocity• Slow Transfer• Bulk of Sample and

Solvent to Column• Many Factors

Important

Steps in a Splitless Injection

• Purge valve is off; column is cold• Inject sample

– fast autosampler injection best– slower injections have been proposed

• Flow through inlet is slow; slow transfer to cold column

• After 30-60 sec, open purge valve - cleans inlet• Temperature program column

BAND BROADENING

• Time

• Space (solvent effect)

• Thermal Focusing

Grob, K., Split and Splitless Injection in Capillary GC, Huthig, 1993, pp. 19-29, 322-36.

Time

Space

Focusing

Band Focusing Mechanisms

• Splitless injections involve slow transfer to column ---> initial peaks are broad

• Need focusing– cold trap– solvent effects

Cold Trap

• Initial column temperature cold enough to “freeze” analyte on column

INITIAL COLUMN TEMPERATURE

40oC 20oC 0oC

-20oC -40oChexane, heptane500 ppb10 min extractionFiber: PDMS 100 mLinermmoCPinj: 1 bar(g)

Solvent Effects

• Solvent is recondensed in the column

• Long plug of liquid

• Start column 30-50 degrees below normal boiling point of solvent

Solvent Effects

Solvent Effects

• Refocus moderate volatility compounds near column head

• Require solvent to wet stationary phase

• Use non-polar solvent with non-polar stationary phase, etc.

INITIAL COLUMN TMPERATURESOLVENT EFFECT INJECTIONS

0 20Time (min)

0 20Time (min)

40oC 60oC

Solvent: Cyclohexane (bp 81oC), Sample: 10ppm hydrocarbons

INLET TEMPERATUREREALITY

Set Point 350oC

DistancefromSeptum(mm)

Carrier Gas Temperature (oC)

Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, 1991, p. 42.

INLET TEMPERATURECHROMATOGRAMS

70000

40000

250oC

100oC

1. octane2. decane3. tridecane4. tetradecane5. pentadecane

HP 5890-5972Pinj = 5.0 psiHP5 30m x 0.25mmx 0.25 mmTransfer: 280oC

1

2

3 4 5

TP: 40oC initial, 1 min, 10oC/min

INLET PRESSURE

• Linear Gas Velocity IncreasedInjectorColumn

• Analyte Boiling Point Increased

PRESSURE PULSE

• Increased Pressure During Injection Only

Time (min)

Pressure(kPa)

50

150

0.75

Purge “ON” Time

20

PRESSURE PULSE

No Pulse

10 psi pulse

12

3 4 5 1. octane2. decane3. tridecane4. tetradecane5. pentadecane

HP 5890-5972Pinj = 5.0 psiHP5 30m x 0.25mmx 0.25 mmTransfer: 280oC

Pressure increased to 15 psig during splitless period

TP: 80oC initial, 1 min, 10oC/min

20000

40000

OPTIMIZATIONSPLITLESS INJECTION

• Can Be Difficult

• Minimize Transport Time (high linear velocity)

• Maximize Thermal Focusing (low initial column temperature)

• Maximize “solvent effect” (low initial column temperature)

• Chemistry remains a factor

REFERENCES

• Grob, K. Split and Splitless Injection in Capillary GC, 3rd. Edition, A. Huethig, 1993.

• Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, 1991.

• Stafford, S.S., Electronic Pressure Control in Gas Chromatography, Hewlett Packard, 1993.